1. Technical Field
The present disclosure relates to a method of treating zeolite, and particularly to a method of calcining a zeolite to remove organic templating agent therefrom.
2. Background of the Art
Zeolites and molecular sieves are generally used in a wide variety of catalytic procedures. In general, zeolites and molecular sieves may be prepared by a procedure which involves forming the structure from a reaction mixture that includes silica and alumina, and often with an organic structure directing agent (often referred to as a “template” or “templating agent”) such as, but not limiting to linear amines, linear diamines, and quaternary ammonium salts. As an example, such quaternary ammonium salt may be tetraethylammonium hydroxide. The organic structure directing agent can be removed from the resultant zeolite by a heat treatment process, often referred to as “calcination”, at an elevated temperature. The acid form of the formed zeolite structure or molecular sieve is then produced by ion exchange, such as, but not limited to, ammonium exchange, followed by further calcination. In some processes, the ammonium exchange step occurs before the calcination, thereby simplifying the sequence of steps. In many cases, the (additional) heat treatment, also referred to as calcination, is executed subsequent to a forming step. In this forming or shaping step, the zeolite or molecular sieve is produced into a shape to allow use in for example fixed bed catalytic operation.
In prior methods of calcining the zeolite is heated in tray calciners or in rotary calciners to remove the templating agent. However, the known methods suffer from drawbacks. In particular, it is possible to develop hot spots of uneven temperature. These hot spots result in a non-uniform organic decomposition and low activity product. Moreover, decomposition or combustion of the organic template can produce water vapor, which results in steaming in certain regions. Steaming can result in localized dealumination of the catalyst, with a change in acid sites and catalyst activity. Such irregularities can adversely affect catalyst performance in certain conversion processes.
In the current art, it has been recognized that the state or characteristics of the zeolite or molecular sieve may be effected by the final heat treatment step. However, it has not been recognized that, in the heat treatment to remove the organic structure directing agent, the performance of the zeolite or molecular sieve is affected significantly by changing the state or characteristics of the zeolite or molecular sieve materials.
U.S. Pat. No. 5,258,570 teaches that the catalytic activity of zeolite beta can be improved by activating the formed zeolite by heating at elevated temperatures of from about 600° C. to 675° C. in order to reduce so-called “strong” acid sites. In accordance with U.S. Pat. No. 5,258,570, zeolite beta produced by conventional procedures is specifically treated to reduce acid sites to thereby increase catalyst activity.
Contrary to what is taught in U.S. Pat. No. 5,258,570, it has surprisingly been found that controlled heat treatment or calcination to remove the organic structure directing agent and exposure of the zeolite or molecular sieve during this treatment to average temperatures preferably below 600° C. is desired to create acid sites of a specific nature and strength. These created acid sites, as can be measured by the temperature controlled desorption of ammonia, are surprisingly found to significantly enhance catalytic performance in reactions, such as, but not limited to, hydrocarbon conversion technologies, and environmental abatement technologies.
What is needed is a method of catalyst preparation which avoids the drawbacks of prior calcining methods in order to achieve better catalyst performance.